It seems these days Donald Trump is ruffling the feathers of more than a few in Hollywood.

The Celebrity Apprentice star, real estate mogul and possible contender for the Republican presidential nomination has upset several celebrities over his "birther" comments related to President Barack Obama, calling his birth certificate into question.

Here are seven who have had harsh words for the outspoken billionaire:

1. Jerry Seinfeld: The actor-comedian last week pulled out of a charity event hosted by one of Trump's sons. Seinfeld had signed on for a Sept. 13 Eric Trump Foundation benefit aiding the St. Jude's Children's Research Hospital, but his manager said the comic has become “increasingly uncomfortable” with Trump’s recent questioning of Obama’s citizenship so he "has respectfully withdrawn from the event." In turn, Trump fired off a nasty letter to Seinfeld. "What I do feel badly about is that I agreed to do, and did, your failed show, 'The Marriage Ref,' even though I thought it was absolutely terrible," Trump wrote. "Despite its poor ratings, I didn't cancel on you like you canceled on my son and St. Jude. I only wish I did."

2. Robert De Niro: The actor, who campaigned for Obama in 2008, also took issue with Trump's birther comments. During a public interview Saturday with NBC News' Brian Williams at the Tribeca Film Festival, which De Niro co-founded, the actor grew passionate while talking about politics. "I won't mention names, but certain people in the news the last couple weeks, just, what are they doing?" De Niro said. "It's crazy. They're making statements about people that they don't even back up. Go get the facts before you start saying things about people." After Williams got De Niro to admit he was referring to Trump, De Niro added: "It's like a big hustle; it's like being a car salesman. Don't go out there and say things unless you can back them up. How dare you? That's awful to do. To just go out and speak and say these terrible things? Unless you just wanna get over and get the job. It's crazy." On Monday, Trump fired back at De Niro on Fox & Friends: "I like his acting, but in terms of when I watch him doing interviews and various other things, we’re not dealing with Albert Einstein. He can say what he wants but the fact is that this guy has not revealed his birth certificate, a lot of people agree with me."

3. Lawrence O'Donnell: Last week, the MSNBC host called Trump "the racists' greatest hero" over his birther stance. According to the Huffington Post, O'Donnell argued that Trump "must realize by now" that, while some of the "hatred" Ameicans feel toward Obama is policy-based, "probably even more of it ... is based on simple old-fashioned racism." He added: "Millions of this country's most virulent racists now feel they have someone they could vote for for president. ... They hope that he is every bit as racist as they are. ... Donald Trump may not be a racist, but he is now the racists' greatest hero. He is their frontman."

4. Whoopi Goldberg: Trump angrily debated with the hosts of The View last month about Obama not being an American citizen. (He said he didn't need to talk about The Celebrity Apprentice because "ratings are through the roof.") "Why doesn't he show his birth certificate? I wish he would," Trump said on the ABC talk show. Co-host Goldberg became increasingly agitated, rolling her eyes and sighing loudly. "It's the biggest pile of dog mess I've heard in ages!" she said. "It's not because he's black, is it? I've never heard of any white president who had to show his birth certificate. That's BS!" Trump later called her comments "insulting" and said his opinion had nothing to do with race.

5. Carol Costello: Following controversy last month about Trump's own birth certificate -- he presented a hospital birthing record, not an actual birth certificate -- the CNN anchor called Trump a "joke" and his birther arguments "ridiculous." She said Trump "is not likely to become president, despite what the polls say" and is just "distracting from an important debate that should be taking place in this country."

6. Glenn Beck: The conservative TV personality, who recently announced he is ending his Fox News Channel show, appeared on FNC's The O'Reilly Factor earlier this month to discuss Trump's presidential aspirations. "He's made me a little uncomfortable here recently," Beck said, adding that "the last thing the country needs is a showboat." He also took issue with the extremity of Trump's birther claims. "There is something reasonable to, you know, a reasonable conversation if you don't believe that he has a birth certificate," Beck said. "You can say, 'I don't believe he has a birth certificate.' But then he goes into 'I do not believe he has a birth certificate' and then he releases one that's worse than Obama's. And then he comes back and he says, on your program, 'He might be a Muslim too.' I'm like, come on!"

7. Michael Bloomberg: On Sunday, New York's mayor implored Trump to drop the birther issue and instead "focus on the main issues." "Look, anyone can run for president if you're 35 and you're an American citizen and born here," Bloomberg said on Fox News Sunday, as reported by the New York Daily News. "The President was born here." Bloomberg -- a former Democrat who now considers himself an independent and a friend of Trump's -- also admonished all Republicans over the birther issue. "I think the Republicans are making a terrible mistake in making this a big issue," he said. "We have immigration. We have the deficit. We have the economy. Those are the things that the public cares about."